You might think it's just a boarding pass, but it possesses a lot of power.
From identifying the passenger to providing the flight number the boarding pass can produce a lot of information.
However, there is one code, "SSSS", that you don't want on your pass.
If you receive a boarding pass with the code SSSS (Secondary Security Screening Selectee) on it, you won't be able to print your boarding card off at an automated machine.
Instead, you have to check-in at the desk where you will be asked more thorough questions than other passengers and undergo less cursory perfunctory identity checks.